title: Mechanistic studies of water electrolysis and hydrogen electro-oxidation on high temperature ceria-based solid oxide electrochemical cells.
abstract: Through the use of ambient pressure X-ray photoelectron spectroscopy (APXPS) and a single-sided solid oxide electrochemical cell (SOC), we have studied the mechanism of electrocatalytic splitting of water (H2O + 2e(-) → H2 + O(2-)) and electro-oxidation of hydrogen (H2 + O(2-) → H2O + 2e(-)) at ∼700 °C in 0.5 Torr of H2/H2O on ceria (CeO2-x) electrodes. The experiments reveal a transient build-up of surface intermediates (OH(-) and Ce(3+)) and show the separation of charge at the gas-solid interface exclusively in the electrochemically active region of the SOC. During water electrolysis on ceria, the increase in surface potentials of the adsorbed OH(-) and incorporated O(2-) differ by 0.25 eV in the active regions. For hydrogen electro-oxidation on ceria, the surface concentrations of OH(-) and O(2-) shift significantly from their equilibrium values. These data suggest that the same charge transfer step (H2O + Ce(3+) <-> Ce(4+) + OH(-) + H(•)) is rate limiting in both the forward (water electrolysis) and reverse (H2 electro-oxidation) reactions. This separation of potentials reflects an induced surface dipole layer on the ceria surface and represents the effective electrochemical double layer at a gas-solid interface. The in situ XPS data and DFT calculations show that the chemical origin of the OH(-)/O(2-) potential separation resides in the reduced polarization of the Ce-OH bond due to the increase of Ce(3+) on the electrode surface. These results provide a graphical illustration of the electrochemically driven surface charge transfer processes under relevant and nonultrahigh vacuum conditions.
authors.affiliation: Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742, USA.
authors.forename: Chunjuan
authors.initials: C
authors.lastname: Zhang
authors.forename: Yi
authors.initials: Y
authors.lastname: Yu
authors.forename: Michael E
authors.initials: ME
authors.lastname: Grass
authors.forename: Catherine
authors.initials: C
authors.lastname: Dejoie
authors.forename: Wuchen
authors.initials: W
authors.lastname: Ding
authors.forename: Karen
authors.initials: K
authors.lastname: Gaskell
authors.forename: Naila
authors.initials: N
authors.lastname: Jabeen
authors.forename: Young Pyo
authors.initials: YP
authors.lastname: Hong
authors.forename: Andrey
authors.initials: A
authors.lastname: Shavorskiy
authors.forename: Hendrik
authors.initials: H
authors.lastname: Bluhm
authors.forename: Wei-Xue
authors.initials: WX
authors.lastname: Li
authors.forename: Gregory S
authors.initials: GS
authors.lastname: Jackson
authors.forename: Zahid
authors.initials: Z
authors.lastname: Hussain
authors.forename: Zhi
authors.initials: Z
authors.lastname: Liu
authors.forename: Bryan W
authors.initials: BW
authors.lastname: Eichhorn